The present invention generally relates to an integrated circuit. The present invention more particularly relates to a comparator circuit for use in an integrated circuit which includes a voltage threshold stage for establishing a threshold voltage and a voltage level generator for providing a voltage which is always sufficient to overcome the threshold voltage. The invention still more particularly relates to such a comparator circuit wherein the voltage level generator provides first and second control voltages, the first control voltage greater than the threshold voltage and the second control voltage less than the voltage threshold. The voltage level generator and threshold stage are arranged to insure that the threshold voltage remains intermediate the first and second control voltages levels notwithstanding variations in integrated circuit processing parameters or integrated circuit operating temperatures.
Voltage comparator circuits for determining which of two voltages is greater are well known in the art and find many applications. Such circuits find considerable use in integrated circuits either alone or in other forms of circuits such as three-state buffers. Three-state buffers are circuits which provide a first output, such as a logical one, when a first input voltage is greater than a second input voltage, and a second output, such as a logical zero, when the second input voltage is greater than the first input voltage. Such circuits generally include an enable input which causes the buffer output to be disconnected from other circuits in response to a disable voltage being applied to the enable input.
In one implementation of such a three-state buffer, the comparator portion includes a threshold circuit which establishes a threshold voltage and a voltage generator which provides first and second control voltages. The threshold voltage is intermediate the first and second control voltages which are applied to a pair of threshold stage inputs in response to the input voltages. The threshold circuit controls the comparator output to be a logical one or zero depending upon which threshold input receives the first control voltage and which threshold circuit input receives the second control voltage. In order for the comparator to perform its function reliably, it is necessary that the threshold voltage remain intermediate the first and second control voltages.
Unfortunately, in the past, integrated circuit processing parameters and integrated circuit operating temperatures have had a strong influence upon whether or not the threshold voltage remained intermediate the first and second control voltages. This resulted because the threshold circuits and voltage generators were configured so that the threshold voltages were permitted to vary independently of the first and second control voltages as a result of varying integrated circuit processing parameters and integrated circuit operating temperatures. As a result, under certain processing or temperature conditions, a threshold voltage became greater than either the first or second voltage levels and, under other such conditions, the threshold voltage became less than either the first or second voltage levels. In either condition, the comparator circuit does not function properly.
The present invention overcomes the aforementioned problems associated with prior art comparator circuits by providing a threshold stage and a voltage level generator that provides a threshold voltage and the first and second control voltages which vary in the same direction and amount in response to variations in integrated circuit processing parameters or integrated circuit operating temperatures. As will be seen hereinafter, this is accomplished by forming these circuits from bipolar transistors which are forward biased in operation, and wherein the forward biased base to emitter voltage drops of these devices are used to advantage to derive the threshold voltage and the first and second control voltages. As a result, the threshold voltage and the first and second control voltages are all dependent upon the forward biased base to emitter voltage drops of the devices so that the threshold voltage will always remain intermediate the first and second control voltages under varying processing or operating temperature conditions.